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australia:keeping_warm

keeping warm when camping or hiking

Introduction

  • hypothermia is a major risk for hikers and campers in cold conditions especially if they are under-prepared, get wet or they have to stop moving due to having to rest, sleep or from injury
  • every 10kph increase in wind speed results in approximately 2degC colder apparent temperature1)
  • cognition becomes impaired when body temperature falls below 35degC and this is likely to create a vicious cycle of further hypothermia as the person no longer makes good judgements to protect themselves
  • shivering is an important survival mechanism when the body gets cold and is maximal when core temperature reaches 35degC but shivering ceases when the body temperature falls below 31degC in which case death is likely to ensue as temperature continues to fall without the protective capacity of shivering.

Hypothermia due to immersion in cold water

Exposure time for core temperature to fall to 35.5°C

Water temp ankle deep knee deep waist deep neck deep
10-12degC 7hrs 5hrs 1.5hrs 5 minutes
13-15degC 8hrs 7hrs 2hrs 5 minutes
15.5-18degC 9hrs 8hrs 3.5hrs 10 minutes
18-20.5degC 12hrs 12hrs 6hrs 10 minutes
>21deg C no limit no limit no limit 30 minutes

NB. times are half that (except for neck deep) if it is raining; those with less body fat or higher surface area may have less times;

Keeping warm

  • be prepared
    • even on short day hikes, no one expects to get lost, injured or caught in an unexpected storm
    • be sensible and take precautions pending the risks of your activity
    • take wet weather gear and some form of shelter even if it is just an ultralight tarp and bivvy bag
    • take a map and compass and a radio beacon (EPIRB)
  • protect against frostbite
    • frostbite (skin death) usually occurs when skin temperature falls below below −2°C, lesser degrees may cause chilblains
    • frostbite to the exposed face in an otherwise properly dressed person is likely to occur when Wind Chill apparent temperatures fall below minus 25degC which would equate to 50kph winds with ambient temps at minus 14degC
    • frostbite to fingers and toes will occur at much less extreme cold conditions
    • when apparent temperatures drop below 4degC, cover your skin especially your feet and hands and keep them dry
    • take a spare pair of gloves, etc in case they get wet
    • consider chemical pocket warmers
    • if frostbite occurs:
      • do not re-warm the skin if there is risk it will freeze again as repeat warming-freezing cycles cause more tissue loss than delayed re-warming
      • if further freezing is not expected, rapidly rewarm by immersing in 39-42degC water
      • start one aspirin per day
      • do not drink alcohol or smoke
      • elevate the affected part
      • manage as per burns
  • don't get wet
    • many fabrics such as cotton and down loose much of their insulating properties when they get wet - generally avoid hiking with cotton or denim, and don't get your down gear wet! Consider synthetics if getting them wet is likely as synthetics generally retain their insulating properties better and they tend to dry out faster. A wet canvas swag will act as a cooling fridge and will take a long time to dry out!
    • don't wear your full arm length puffer jackets while hiking unless it is extremely cold - you will just perspire too much - wear layered breathable clothing even if its raining
    • dry off as soon as possible
    • avoid perspiration before going to sleep - avoid exertion, excessive clothes (add layers as temperatures drop)
    • keep wet gear outside the tent
    • don't sleep with your head inside the sleeping bag - this will make your sleeping bag wet from your exhaled breaths
    • minimise and manage condensation in tents
  • keep thermal wear clean as well as dry
    • dirt impairs insulating properties
  • don't get injured in cold weather
    • this will impair you ability to keep warm by moving and to seek shelter
  • have a shelter from the rain and wind
    • whilst tents are poor thermal insulators, in addition to protection from rain and wind chill, they can create a microclimate by helping to retain some body heat - especially if the tent is small and the canopy is full fabric rather than mesh
    • a 2-man tent fly with minimal wind will provide 0.4 to 1.4deg warmer microclimate than outside
    • a mesh canopy combined with a tent fly will probably be marginally warmer than a fly alone as the mesh does reduce wind flow
    • a 2-man fabric canopy tent with fly and mesh doors with minimal wind will provide approx 4 deg warmer by the end of the night microclimate than outside
    • a 2-man fully enclosed fabric canopy tent (apart from opposite ceiling vents) with fly and fabric doors with minimal wind will provide approx 5-6deg warmer by the end of the night microclimate than outside and probably warmer than this if occupied by 2 people. It will probably give at least 3-4deg extra warmth (and much more effective warmth if there is wind chill in the mesh tent) towards the end of the night compared to an all mesh tent with fly - thus you need to compare the weight savings of a mesh tent with the extra weight (perhaps 250g) of a thermal liner for extra sleeping bag warmth, although a liner does not help with the inspired air temperature
  • reduce your heat losses:
    • don't get sunburnt during the day - this will increase heat loss from inflamed red skin
    • don't drink too much alcohol - this will increase heat loss from skin as it opens up the skin blood vessels when they should be closing down
    • cover your skin including head, feet and hands
      • generally layered clothing is best so that you can regulate your warmth without getting too hot and sweaty
      • thin merino long arm tops and long johns are popular undergarment layers for cold weather conditions
    • use an insulating sleeping pad - much of your heat loss during sleep is via conductive losses to the cold ground, this also applied when sleeping in a hammock
    • use an appropriate sleeping bag or quilt with appropriate thermal clothing layers as needed without causing perspiration
    • sleep in the least amount of clothes that will stop you feeling cold
      • have additional layers inside your sleeping bag to keep them warm should you need to layer up during the night as the temperature drops
    • don't sleep in your socks or underlayer (eg. thermal top/longjohns) you have just hiked in - have a clean warm pair of socks and underlayer reserved for sleeping
    • consider zipping up your jacket and slipping it over the bottom end of your sleeping bag to give your feet an extra layer of warmth
    • consider putting your DRY, CLEAN clothes in your sleeping bag so they are not so cold when you put them on the next morning and put anything unclean such as boots in a plastic bag inside the tent so they are not freezing in the morning
    • reduce your surface area by curling up into a fetal position and have hands between thighs
    • in very cold climates, have a pee bottle - saves you getting out of the tent at night and the bottle will keep you a little warmer, alternatively, ensure you have some sandals to slip on to go out rather than needing to put your boots on
  • add extra heat
    • warm food and drinks
    • radiant heat from a fire outside your tent
    • hot water bottle such as Nalgene bottle placed in a sock (or perhaps warm stones from the fire)
    • go for a walk or do gentle exercise prior to sleep to generate some muscle heat but without creating perspiration
    • sleep with a partner
    • consider additional heat sources such as:
      • USB-powered heating pads
      • 12V car seat heater cushions rated at 45W are generally around 12W (1A) averaged and will raise temperature by around 2-4degC as well as providing direct heat
      • 12V 130W 10.5A air heater will raise temp from 13 to 19degC within 10 minutes in a full fabric 2 man tent
      • NB. each 10Ah 12V LiFePO4 battery capacity will weigh at least 1.1kg (see battery systems for camping)
  • carbon monoxide poisoning is a major risk in poorly ventilated enclosed spaces such as tents if fuel is burnt inside or nearby
    • carbon monoxide (CO) is a colorless and almost odorless gas that results from the incomplete combustion of hydrocarbon fuels when oxygen levels are to low (for LPG, the ratio of air to propane needs to be greater than 24:1 to avoid CO production)2)
    • levels of CO above 100ppm create risk as poisoning is cumulative with duration of exposure
    • levels above 800ppm may result in unconsciousness within 2 hours and you may not wake up to extract yourself and ongoing exposure may result in death
    • levels above 1600ppm (1.6%) may result in death within 2 hours
    • levels above 1-2% may result in death after only a few breathes
    • the effects are greater at high altitudes above 3,000m where oxygen levels are low
    • smoke from a wood fire usually has around 5000ppm CO - does not require much to get into a small tent to be lethal!
    • for gas stoves with incomplete combustion, the rate of CO production depends upon two factors: the amount of fuel burnt by the stove and the degree of incomplete combustion.
    • running a small camp gas stove in an A-frame tent with small vents for 15 minutes resulted in CO levels of stabilising around 100-130ppm as production equals ventilation losses of CO, especially if the flames hit the bottom of the cooking pot - elevation of the pot by 25mm significantly reduced incomplete combustion; 2hrs of use at these levels would cause 5% reduction in oxygen saturation of Hb which could be critical at high altitudes 3)
  • if you do decide to use a gas heater in a well ventilated tarp or tent space make sure you have a CO detector alarm!

Mechanisms of heat loss from the body

respiration

  • the body is constantly losing heat by warming up and humidifying the inspired air
  • this can be reduced by warming the air before we inspire it (eg. sleep in an enclosed tent rather than under a tarp)

evaporation

  • when moisture evaporates it uses heat to do so and thus cools the skin in the process
  • this is the main mechanism we manage high temperatures but it also becomes a very important factor in creating hypothermia if we are cold and wet (eg. from perspiration whilst exercising or becoming too warm)
  • it is also the reason why reducing condensation in tents is important so you don't get wet while sleeping

convection

  • if the air layer close to your body is constantly changing, your body loses heat as it is constantly heating up the air layer
  • this is why wind chill is such an important factor
  • wind protection such as layered wind-resistant clothing, a tent and sleeping bag all contribute to reducing convective losses
  • convection loss is dependent upon:
    • area of skin exposed and the degree of insulation over that skin
      • your head accounts for 10% of the body's surface area but becomes a much higher proportional source of heat loss when the rest of your body is insulated by clothing but your head is not - hence in the cold, wearing a beanie or similar is important in reducing your heat losses
    • wind chill: air temperature and air speed but further compounded if skin is wet

radiation

  • all things radiate heat at a rate to the 4th power of the temperature in degrees Kelvin
    • rate of heat loss = constant x surface area x temp in Kelvin4
  • you can retain some of this by use of reflective materials around your body such as “space blankets” and insulating clothing to retain this heat near the skin
  • if you are trying to warm yourself near a fire, wearing layers of insulation will dramatically reduce the radiant warming effect of the fire so one needs to weigh up the pros and cons of clothing with regard to its protection from wind chill etc vs impairing heat gain from the fire

conduction

  • heat flows from a high temperature source (your body) to a lower temperature area (eg. the ground you are lying on and the air layer close to the skin, or worse, very conductive materials such as cold metal close to the skin)
  • the rate of this heat loss is dependent upon:
    • the temperature gradient (this would be equivalent to voltage differential in electric currents)
    • insulation factors (this would be equivalent to resistance in electric currents)
  • this is why an insulating sleeping pad is important when camping

urination

  • this is a less important source of heat loss however if you drink a lot then you will urinate a lot and the urine will be at body temperature so you are losing heat from the body
  • avoid drinking lots of alcohol!
  • but drink enough water to stay hydrated

Mechanisms of thermoregulation of the body

perspiration if too hot

  • humans have evolved an important mechanism of perspiration to increase evaporative heat loss in hot conditions
  • this is less effective when it is hot and humid

hyperventilation if too hot

  • this is why hairy animals which cannot use perspiration as significant form of heat loss (eg. dogs) need to pant in hot weather

peripheral vasospasm of skin if too cold

  • reducing blood flow to the skin reduces heat loss from the skin but also risks frostbite
  • excessive alcohol can impair this as alcohol is a vasodilator and thus increases risk of hypothermia

shivering if too cold

  • shivering generates heat from high levels of muscle work
  • shivering occurs when the hands or feet get cold and this sends a signal to the brain to commence shivering
  • maximal shivering generally occurs when core temperature reaches 35degC and ceases when it falls below 31degC in which case death is likely to ensue as temperature continues to fall.

exercise if too cold

  • the risk of hypothermia significantly increases when you stop walking

digestion generates heat

  • the process of digesting food generates heat
  • eating food before going to bed (especially if warm as this adds body heat in itself) not only creates heat from digestion but provides energy for shivering

reflective emergency space blankets

  • cheap Mylar ones are crinkly and noisy and not durable, so better options are below
  • SOL 1-2 person heatsheet Emergency Blanket
    • 1.52×2.44m reflective 1mill tarp/blanket
    • 95g $AU15
  • AMK Nano Heat Travel Blanket
    • 170x147cm; 193g; quiet, soft, reflective fabric $AU66
  • SOL Heavy Duty All purpose Emergency Blanket (can be used as tarp)
    • 1.52×2.44m reflective 2.5mill tarp/blanket
    • 223g $AU31 or included in the SOL Emergency Shelter Kit with paracord and pegs for $AU53

thermally insulated sleeping mats

  • avoid thick air mattresses as they tend to drain warmth from you
  • aim for thermally rated sleeping pads

sleeping bag temperature ratings

  • assume sleeping on a sleeping mat and wearing base thermal layers
  • women's bag just use comfort level (comfortable posture and not shivering) if only one quoted
  • men's bags use the colder “lower limit” or “transition” level (curled up but not shivering) if only one quoted
  • “extreme level” is emergency survival only - you will be curled up and shivering even with layers on
  • If a bag is too small, you’ll end up compressing insulation and creating cold spots.
  • if a bag is too big, you spend some of your energy heating empty space. For very cold conditions, many prefer a mummy style bag to reduce empty space at the feet end at the expense of reduced freedom of movement.
  • it’s important not to get a bag that’s too warm; if you sweat, you’ll wake up cold.
  • avoid putting heavy layers on top of a down bag as it will compress the down and lose benefits
  • A +5 to +10 is considered a summer bag. It is roughly as warm as sleeping with a sheet or light blanket over you on your bed at home.
  • A 0 degree bag is a great all-rounder but won’t keep you warm in the snow or frosty night – it’s about the same as having a regular doona on your bed at home.
  • A -5 to -10 degree bag is considered a winter bag for typical Australian climates – it should be similar to having flannelette sheets and an extra blanket or 2 on your bed at home.

sleeping bag liners

  • thin silk or cotton liners may add a few degrees warmth
  • a thermal liner weighing 250g may add an effective 8 degrees to your sleeping bag rating

keeping your tent extra warm on very cold nights

  • use the smallest tent suitable for your needs - it is easier to keep warm
  • use a full fabric tent rather than a mesh tent, especially one with dual fabric/mesh door options
  • choose a sheltered camp site
    • if camping in snow, create a snow wall shelter from the wind
  • position your tent appropriately for the wind direction (although remember it can change overnight!)
  • close the vestibules
    • this will result in the air temperature outside your tent but inside your fly being 0.4-1.4deg warmer than outside and thus creates a better temperature gradient for your tent as well as reducing wind chill and reduce loss of warm tent air through excessive ventilation on windy nights
  • ground insulation
    • consider an extra waterproof ground sheet (eg. tarp) and thermally insulated padding under the tent (eg. foam rubber thick mats)
    • consider using corrugated cardboard as this will also absorb moisture
    • add a space blanket on the tent floor with reflective layer upwards and if possible tape it to the sides of the tent 12-15cm above the floor
  • top insulation
    • add a space blanket
      • duct tape or fasten the blanket all over the top of the inside of your tent with the aluminium of the blanket facing the inside.
    • consider extra one or two waterproof tarps over your tent in addition to the fly
  • wall insulation
    • consider lining walls with space blankets or 10mm bubble wrap (insulating rating of R1 and can also be used above bed)
  • consider a heater
    • a USB heater mat with a power bank will add some valuable warmth to your skin (better still, a 12V car heated cushion at around 1A/hr or a 12V electric blanket - assuming you have an auxiliary 12V battery)
    • a heating mat allows you to sleep naked which will add a few degrees extra warmth to the air temperature inside the tent if it is a full fabric tent and can retain the warm air
australia/keeping_warm.txt · Last modified: 2021/08/22 11:46 by gary1